312
A. Latini et al. / Journal of Alloys and Compounds 346 (2002) 311–313
powder (Aldrich, 99.9% pure), according to the reaction:
the reaction and elimination of the phases thermody-
namically less stable by their decomposition. XRD patterns
support these arguments. The pattern in Fig. 1A corre-
sponds to the sample taken before annealing. It shows that
the PrB6 phase has well been formed and is crystalline,
even if there are still some weak peaks which do not
belong to this compound (see insert in Fig. 1A). However,
in the pattern shown in Fig. 1B, corresponding to the
annealed sample, these peaks have disappeared (insert Fig.
Pr6O11(s) 1 47B(s) → 6PrB6(s) 1 11BO(g)
The mixture was thoroughly ground for 10 min with the
addition of some milliliters of dry diethyl ether. The
mixture was transferred into a boron nitride crucible which
was then positioned into a water cooled electron beam gun
(Ferrotec mod. EV1-8, Germany). The e-beam gun is
positioned into a stainless steel vacuum chamber evacuated
by a turbo pump. The vacuum system was switched on
and, after a few minutes, the working pressure of 531025
mbar was reached. Then the electron beam bombardment
system was activated. The accelerating voltage used during
the experiment was 23.5 kV. Initially, for 5–10 min, the
powder was outgassed with a 1-mA large spot; then, the
emission current was gradually increased to reach 150 mA
and this treatment continued for 30 min. Subsequently, the
reactants were melted with a 100–130-mA point shape
spot. The time necessary to melt the reactants was a few
minutes. Irradiation was then stopped, the chamber vented
with N2, and the reactants remixed. The whole procedure
was repeated three times.
After the last treatment, the reaction product, which
appears as beautiful brilliant electric blue fused lumps, was
ground and a sample was taken for a first XRD analysis;
the rest of the powder was annealed using e-beam bom-
bardment for 1 h (large spot, 100–130 mA). After anneal-
ing, another XRD analysis was carried out. It is important
to point out that it is necessary to avoid melting of the
reactants from the beginning because the evolution of
gaseous BO can become explosive with large losses of
material.
XRD analysis was performed by a Philips X’Pert
powder diffractometer using Cu Ka1 radiation.
Peak positions and lattice parameters were calculated by
the bundled software.
3. Results and discussion
Remixing of the reactants is necessary because the
bottom and the walls of the crucible, which are in contact
with the body of the gun, are cold. No interaction was
observed between the crucible and the reaction product.
Electron beam bombardment permits extremely high tem-
peratures to be reached and thus it is possible to overcome
problems due to solid state diffusion barriers which can be
the rate limiting step in this type of reaction. This method
allows the synthesis in a relatively short time (4–5 h total
work, including the time necessary for evacuating and
venting the chamber) of refractory materials such as
carbides and borides which are suitable for preparation in
vacuum unlike nitrides or oxides. The possibility of
attaining high temperatures, together with control of the
position and shape of the beam, allows the completeness of
Fig. 1. (A) X-ray diffraction pattern of PrB6 before annealing. Magnifica-
tion is given in the insert. The black arrows indicate weak peaks that are
not assignable to PrB6. (B) X-ray diffraction pattern of PrB6 after
annealing. Magnification is given in the insert. The weak peaks not
assignable to PrB6 have disappeared.
Latini, Alessandro
